A quantitative histologic analysis of ventricular septal architecture was undertaken at necropsy in a large population of infants with normal or diseased hearts (includng aortic or pulmonic valve atresia) and compared with patients having hypertrophic cardiomyopathy. Disorganization of cardiac muscle cells occurred more commonly and to a significantly greater extent in patients with hypertrophic cardiomyopathy. It is most likely that extensive cardiac muscle cell disorganization in the ventricular septum is a morpologic manifestation of the genetic defect in hypertrophic cardiomyopathy.